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SubjectRe: KASAN: use-after-free Write in hiddev_disconnect
On Tue, 14 Jan 2020, Dan Carpenter wrote:

> I'm trying to take a stab at diagnosing these bugs. (But I'm seldom
> smart enough to actually fix anything). These hiddev_disconnect() bugs
> are a race condition:
>
> devel/drivers/hid/usbhid/hiddev.c
> 924 void hiddev_disconnect(struct hid_device *hid)
> 925 {
> 926 struct hiddev *hiddev = hid->hiddev;
> 927 struct usbhid_device *usbhid = hid->driver_data;
> 928
> 929 usb_deregister_dev(usbhid->intf, &hiddev_class);
> 930
> 931 mutex_lock(&hiddev->existancelock);
> 932 hiddev->exist = 0;
> ^^^^^^^^^^^^^^^^^^
> We set "exist = 0;"
>
> 933
> 934 if (hiddev->open) {
> 935 mutex_unlock(&hiddev->existancelock);
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> Then we drop the lock.
>
> 936 hid_hw_close(hiddev->hid);
> 937 wake_up_interruptible(&hiddev->wait);
> ^^^^^^
> The other thread frees hiddev and it crashes here.
>
> 938 } else {
> 939 mutex_unlock(&hiddev->existancelock);
> 940 kfree(hiddev);
> 941 }
> 942 }
>
> The other thread is doing:
>
> drivers/hid/usbhid/hiddev.c
> 216 static int hiddev_release(struct inode * inode, struct file * file)
> 217 {
> 218 struct hiddev_list *list = file->private_data;
> 219 unsigned long flags;
> 220
> 221 spin_lock_irqsave(&list->hiddev->list_lock, flags);
> 222 list_del(&list->node);
> 223 spin_unlock_irqrestore(&list->hiddev->list_lock, flags);
> 224
> 225 mutex_lock(&list->hiddev->existancelock);
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^
> Takes the lock.
>
> 226 if (!--list->hiddev->open) {
> ^^^^^^^^^^^^^^^^^^^^
> Decrements open.
>
> 227 if (list->hiddev->exist) {
> ^^^^^^^^^^^^^^^^^^^
> This is false.
>
> 228 hid_hw_close(list->hiddev->hid);
> 229 hid_hw_power(list->hiddev->hid, PM_HINT_NORMAL);
> 230 } else {
> 231 mutex_unlock(&list->hiddev->existancelock);
> 232 kfree(list->hiddev);
> ^^^^^^^^^^^^
> Freed here.
>
> 233 vfree(list);
> 234 return 0;
> 235 }
> 236 }
> 237
> 238 mutex_unlock(&list->hiddev->existancelock);
> 239 vfree(list);
> 240
> 241 return 0;
> 242 }
>
> I'm not sure what the fix should be though.

Good sleuthing!

It certainly looks as though hiddev_disconnect() should not try to
reference hiddev after dropping the existance_lock -- at least, not in
the case that the device file is still open. So I suggest reordering
the statements in that routine: First call hid_hw_close() and
wake_up_interruptible(), then drop the mutex.

Too bad syzbot doesn't have a reproducer to test this, though.

Alan Stern

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